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Passivating problems

 

We are relatively new to the chemical business and are still going through our learning curve, our problem of the day is passivating 400 series stainless steel without the metal tarnishing or having a "rusty" look. Is diluting the acid a correct way to solve the problem or do we look somewhere else? We have done this in the past, had the same problem but we never know the size order we're getting and the separate mixture of acids isn't the best way for us to go we don't think. Any insight would be appreciated.

Thanks,

Derrell Fowler
- Cullman, AL, U.S.A.

Hello Derrell!

Likely the first thing you want to do here is read the FAQ that Ted and Tom have about passivation. Next, if you do a search of just finishing.com on "passivation" you'll get a lot of letters, some of which bear on your problem. Finally, some of those letters were responded to by folks who work with citric acid passivation: I'd recommend you contact them, and they'll get you on track.

Good luck!

Lee Gearhart
metallurgist - E. Aurora, New York

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Derrell:

Per Lee's comments, there are excellent ways to passivate the 400 series stainless with citric acid without ruining the surface. There are citric acid formulations on the market that will work well. All citric formulations are not acceptable for this. DEFINITELY DO NOT dilute nitric acid to try this. It will make it worse. If you MUST use nitric acid, you will need to add sodium dichromate to the solution. See ASTM A967.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois